The mammalian cochlea achieves its remarkable sensitivity, exquisite frequency selectivity, and enormous dynamic range through an active amplification mechanism called the cochlear amplifier, which uses metabolic energy to boost basilar membrane (BM) vibration. Although it is well established that outer hair cells (OHCs) can generate forces through somatic and hair-bundle motility, the mechanical mechanism of cochlear amplification remains unclear. This application is to study the role of somatic and hair-bundle motility in cochlear amplification by conducting a set of novel experiments in normal and genetically modified cochleae using our recently developed scanning low-coherence heterodyne interferometer. Specific aim one will measure the BM, reticular lamina (RL), and tectorial membrane (TM) vibration in sensitive mouse and gerbil cochleae. The hypothesis is that, in sensitive mouse cochleae, the OHC-based cochlear amplifier produces a larger and more nonlinear vibration at the RL than that at the BM. BM and RL vibrations in mice are highly sensitive, and show sharp tuning and nonlinearity as in other commonly used animals for cochlear mechanics study, such as gerbils. The data from this experiment will be essential for studying cochlear micromechanics in mice and gerbils. Specific aim two will determine the role of the OHC somatic motility by measuring the BM, RL, and TM vibration in mouse cochleae without somatic motility. The hypothesis is that, in prestin 499 knockin mice, the lack of somatic motility results in decreased sensitivity and loss of sharp tuning and nonlinearity of the BM, RL, and TM vibration. Because the mechanoelectrical transduction, stiffness, and morphology of OHCs in prestin 499 knockin mice are normal, the expected changes will be attributed to the absence of somatic motility. Specific aim three will determine the role of the OHC hair-bundle motility by measuring the BM, RL, and TM vibration in mouse cochleae with ineffective hair-bundle motility. The hypothesis is that shortened and detached TM with free-standing hair bundles compromises hair-bundle motility, mechanoelectrical transduction, and cochlear amplification in TectaC1509G/C1509G mice, decreasing sensitivity, sharp tuning, and nonlinearity of acoustically evoked BM, RL, and TM vibration. However, electrical stimulation activates both somatic and hair-bundle motility and results in BM, RL, and TM vibration and electrically evoked otoacoustic emissions. After somatic motility is suppressed by salicylate, hair-bundle motility-mediated responses can be measured in wild-type but not in TectaC1509G /C1509G mice. The contribution of hair-bundle motility to cochlear amplification will be determined by measuring and comparing electrically evoked responses in TectaC1509G /C1509G and wild-type mice after salicylate application. The new data from this study will provide critical information on the essential role of OHC somatic and hair-bundle motility in cochlear amplification and for understanding mechanisms of auditory disorders in humans.